Steam reforming is a method for producing hydrogen from hydrocarbons, such as methane. The basic chemistry of steam reforming uses a temperature-driven reaction of a hydrocarbon with water to produce a “synthesis gas,” a mixture of primarily hydrogen, water, carbon monoxide, and carbon dioxide as well as nitrogen for ammonia synthesis. This synthesis gas is sometimes more generally referred to as a “reformate” in which nitrogen can be just a trace amount of one element.
A “steam reformer” or “burner/reformer assembly” can comprise two flow regions. The first region can provide thermal energy from hot gases, produced, for example, by the combustion of fuel and oxygen, and called the “burner zone.” The second region allows an endothermic steam reforming reaction between fuel and steam, and is called the “reforming zone,” “reformer module,” or “reforming tubes.” These two flow regions are usually physically separated by a heat exchange boundary.
One challenge in steam reforming is transferring enough energy through the heat exchange boundary to sustain the reaction at a desired reaction temperature. The reaction temperature affects hydrocarbon conversion equilibrium and reaction kinetics. Higher reaction temperature in the reforming zone corresponds to a lower heat transfer resistance, higher hydrocarbon conversion, and a lower amount of residual hydrocarbon remaining in the reformate. This reaction can be accelerated by using a catalyst containing a material such as, for example, nickel, a precious metal, or another material containing a special promoter.
High reaction temperatures, however, can cause severe thermal stress, corrosion, creep, and fatigue in metal components of the heat exchange boundary and/or catalyst. Conversely, low reaction temperatures in the reforming zone can reduce metal stress, corrosion, creep and fatigue, and lead to lower hydrocarbon conversions. Higher amounts of hydrocarbons in the reformate can cause difficulties in a subsequent hydrogen separation stage. Furthermore, the more hydrocarbons left in the reformate, the less efficient the steam reformer system becomes. This leads to a higher cost of hydrogen and a higher level of carbon dioxide (a greenhouse gas) emissions per unit of hydrogen produced.
Large scale industrial steam reformers often use multiple reformer tubes as the heat exchange boundary, surrounded by “hot-gas impingement” style burner modules. A burner fuel-air mixture can be fired in the space around the tubes, either directly toward the reformer tubes, along them, or parallel to the reformer tubes from the top and/or from the bottom.
The reforming zones of such steam reformers often operate at high temperature (>850° C.) and pressure (as high as ˜30 bar), running continuously with few startup-shutdown cycles to prolong the usable life of the tubes. To control the temperature profile along the length of the reactor tubes, large industrial reformers sometimes use multiple burner heads along the reformer tubes to avoid the high local temperatures that are typically required if a single burner is used.
Due to the large cost of construction of centralized reforming plants, many economic studies of a hydrogen economy have noted the potential advantages of smaller scale distributed hydrogen production for use in, for example, appliances or other devices. For many applications, the demand of hydrogen is likely to be intermittent (non-limiting examples include a hydrogen fueling station serving a fleet of fuel cell or CNG/H2 capable vehicles to a residential-scale hydrogen refueling appliance, a merchant hydrogen appliance, a reformate production appliance, a combined heat and power (CHP) appliance, and a combined heat, hydrogen, and power (CHHP) appliance). To run efficiently, these hydrogen producing devices must start and stop many times while maintaining their performance and structural integrity. Small scale reformers can generally not afford the expense, space demand, and complexity of staged combustion, and often use a single stage in situ combustion to supply heat to the reforming reaction. Single stage combustion, however, often results in localized high temperatures on the reformer tubes. Frequent startup-shutdown cycles and temperature excursions repeatedly expose reformer components to severe thermal gradients and temperature spikes, both of which cause high thermal stresses, potentially inducing failures in a relatively short period of time. Additionally, heat transfer effectiveness is diminished along the combustion products flow direction on account of their falling temperature (i.e. heat transfer theory provides that the radiative component of heat flux scales with temperature to the 4th power).